Many communication networks that provide high bit-rate transport over a shared medium are characterized by non-continuous, or burst mode, data transmission. An example of such a network is a passive optical network (PON) 100 schematically shown in FIG. 1. A typical PON 100 includes a plurality of optical network units (ONUs) 120-1 through 120-M coupled to an optical line terminal (OLT) 130 via a passive optical splitter 140. Since all ONUs function in like manner, they will be collectively referred to by the reference numeral 120 in the following description unless reference is made to a specific ONU. Traffic data transmission is performed over two optical wavelengths, one for the downstream direction and another for the upstream direction. Thus, downstream transmission from the OLT 130 is broadcast to all the ONUs 120. Each ONU 120 filters its respective data according to, for example, pre-assigned labels. Transmission from an ONU 120 to the OLT 130 is in the form of a burst, hereinafter referred to as a “cell”.
The OLT 130 continuously transmits downstream data to the ONUs 120 and receives upstream burst data sent to OLT 130 from ONUs 120. The OLT 130 broadcasts data to the ONUs 120 along a common channel so that all the ONUs 120 receive the same data. An ONU 120 transmits data to the OLT 130 during different time slots allocated by the OLT 130. That is, the OLT 130 is aware of the exact arrival time of data and the identity of a transmitting ONU 120.
A PON is typically designed with varied lengths of optical links, splits, cost driven optics, and other physical consideration, and thus suffers from optical aberrations influencing the signals. Therefore, appropriate signal processing is required in order to recover the original signal from the received signal and to avoid errors during transmission.
An optical signal sent from an ONU 120 is received by a receiver in the OLT 130 and converted into an analog electrical signal. The OLT's receiver uses a clock and data recovery (CDR) circuit or a burst mode CDR (BCDR) circuit to generate a clock corresponding to the incoming data, thereby correctly retiming the digital incoming data. After recovering the data, a forward error correction mechanism may be utilized to detect and correct errors in the received data and to provide an assessment of the signal quality. However, during the recovery process, essential information, such as eye distortion, frequency movement, phase information, and other effects are discarded, and thus the quality of the input signal cannot be correctly measured. Therefore, assessment of the signal quality is necessary prior to recovering the signals. Specifically, such assessment is required to perform signal diagnostics, per ONU, in order to reduce operational costs by better analysis of link and equipment defects, prevention of communication violations by early detection of deteriorated laser signal quality, and so on.
Measurements of signal quality are typically performed by phase margin measurements, e.g., on an eye pattern diagram. The eye pattern diagram further provides an eye measures on the additive noise and distortions in the signal. Other techniques for determining the level of jitter present at the received signal include measuring phase error parameters of a phase-lock-loop (PLL) or delay-locked loop (DLL).
Prior art solutions for measuring the quality of signal received through an optical line include a dedicated circuit that installed at the front end of a CDR circuit. Examples for such techniques may be found in US2006/0223478 and in U.S. Pat. No. 6,961,520 both incorporated herein by reference for the useful understanding of the background of the invention. These solutions do not utilize the already existing capabilities of CDR circuits for measuring phase errors, as this requires implementing mixed-signal circuits that are difficult to design. In addition, prior art solutions are not adapted to measure the quality of burst signals, and are thus not feasible in PON systems.
Other prior techniques for assessing of signals quality perform power and modulated signal amplitude indication (RSSI) measurements by estimating at least the incoming optical power. However, these techniques do not analyze the pattern of the input signal.
In PON systems there is an increasing demand to perform optical line diagnostics by statistical analysis of the received signals to determine the root cause of failures and enable PON operators the ability to perform optical layer supervision. The optical layer supervision allows more efficient operation and maintenance of PON networks, for example, by not sending technicians if the PON system works properly, dispatching the correct technician if a problem is detected the PON system, or providing correct diagnostics to the technician.
It would be therefore advantageous to provide a method and system for measuring the quality of a burst signal and for performing the root cause analysis of failures in PON systems.